1. Field of the Invention
Stringed musical instrument pickups which provide an electrical signal corresponding to the sound-producing vibrations of a musical instrument by sensing the motions of strings and translating string motions into an electrical signal.
2. Discussion of Prior Art
A primary object of stringed instrument pickup innovation has been to provide a capability of accurately reproducing the entire harmonic spectrum or "voice," of a particular musical instrument. The overall vibrational characteristics of a particular instrument directly and simultaneously influence the relative amplitudes of a string's harmonics and therefore an accurate representation of an instrument's voice can be obtained by sensing only string motions of the particular instrument. However there are many obstacles to achieving this goal, many of which have not been adequately addressed by prior art.
A major consideration which has often been overlooked is that a string's harmonic nodes, and loops located halfway between the nodes, are formed at all equal divisions of string length. Since the harmonic segments overlap each other and change when string length is changed during playing of the instrument, it follows that sensing string motion at a given intermediate location may represent a given harmonic at its loop, or maximum displacement, or at the node where no motion takes place, or anywhere between. Accurate harmonic representation then will not be achieved.
However, since an instrument's bridge serves as a node for the fundamental and all subsequent harmonics it is possible to achieve very accurate representation of all string motions, and hence an instrument's tone, or voice, by sensing string motion at a point close to the bridge. The inherent distortion at such a point is largely an addition of emphasis to succeedingly higher harmonics. This is because the fundamental and lower harmonics will be sensed relatively near their nodes while the higher harmonics will be sensed closer to their loops. This progressive emphasis of higher harmonics can be beneficial to a degree since many components of a pickup and subsequent electronic parts provide the opposite effect of attenuating high frequencies. The curve of succeeding harmonic emphasis is also close to the reverse of the curve of attenuation of simple resistor and capacitor tone controls and therefore additional signal compensation, to resolve the signal to accurate representation, can be easily achieved.
Prior art pickups, however, have not been able to take full advantage of near to bridge locations. One problem inherent in almost all pickups is that they have a largely "open" magnetic field, or one that diminishes along a string only as the square of the distance from the magnetic field source. In this case conflicting motions of a given harmonic may simultaneously be registered in the pickup coil, even arbitrarily cancelling certain harmonics, and so only very "fuzzy" harmonic representation can be achieved and the advantages of a near bridge location are largely lost. Another problem is that only very slight motions of the strings are available near the bridge to act on the pickup components and much of this energy is in the form of high harmonics which are subsequently attenuated. This places the additional requirement, then, that the pickup must be extremely efficient.
Sensitivity to string motion can be increased when the strings pass through the hollow center of a coil, particularly when an individual coil is provided to each string, or in the case of double-strung instruments such as mandolin, to each pair of strings. Prior art in U.S. Pat. Nos. 3,249,677 (Burns), 3,571,483 (Davidson), 3,715,446 (Kosinski), and 3,983,778 (Bartolini), recognize and explain the advantages of providing individual coils to each string. Some of these are: lower impedance, higher resonant frequency, and the possibility of individual treatment of the signal from each string. U.S. Pat. Nos. 2,263,973 (O'Brien), and 2,455,575 (Fender), utilize an elongated coil surrounding an instrument's strings and the latter invention of channel shaped pole pieces also describes an embodiment which provides a separate coil surrounding each string.
But the impedance, capacitance and resonant frequency problems with large coils are destructive of both signal quality and efficiency. And the efficiency requirements with individual coils are great since string spacing often places severe restrictions on maximum outside coil diameter and where the string passes through the coil the inside diameter must be large enough to accommodate string displacements for any type of playing. While there have been many innovations and improvements in pickup design, none of the prior art configurations of magnet and coil pickups are capable of meeting the highly demanding requirements and considerations of fully representing the harmonic spectrum of a stringed musical instrument. The harmonic richness present in the string vibrations of even solid body electric instruments has therefore not previously been made available to musicians. Better reproduction of the tone of acoustic instruments has generally been achieved by sensing vibrational motions of an instrument's top, such as with bridge mounted transducers, or with microphones. These methods, however, have significant inherent problems. Transducer pickups have a very high ambient electrical noise to signal ratio, even when close-to-source preamplification is provided, as well as a tendency to "roll off" high frequencies. A significant problem associated with all such methods is the tendency to acoustic feedback. In this feedback circle sound waves from the speaker return to the instrument's large vibrating surface top and cause an increase in the amplitude of the top's vibrations. This increase is then registered by the pickup causing a stronger signal, and stronger sound waves, repeating the circle and so on. The result is a loud "booming" or other unwanted sounds when a string is plucked. This problem is particularly severe in performance situations where the volume level must be high enough to be heard along with other instruments and in many cases the harmonic richness of an acoustic instrument has been excluded as a possibility to musicians because of this universal problem.